1. Field of the Invention
The present invention relates synchronization in a direct sequence spread spectrum receiver used in radio communication systems, and more particularly it relates to a pseudo noise (PN) code acquisition apparatus for direct sequence spread spectrum communications.
2. Description of the Related Art
A direct sequence spread spectrum communication system is a system in which, in response to an output signal from a transmitter end, which has been spread across a spectrum using a PN code and modulated onto a carrier, a receiver end generates the same PN code as that included in the signal received thereto, and despreads the received signal based on the generated PN code, thereby obtaining the original signal.
FIG. 1 is a block diagram illustrating the configuration of a conventional PN code acquisition apparatus. A spread spectrum signal transmitted from a direct sequence spread spectrum system over a radio channel is received in the PN code acquisition apparatus via a receiving antenna and then down converted. The down-converted received signal, which is denoted by the reference character xe2x80x9cS1xe2x80x9d, is inputted at a despreading unit 12 after passing through a matched filter 11 for obtaining a maximum signal to noise ratio (SNR). The despreading unit 12 despreads the received signal by multiplying the signal by a spread code synchronized therewith so that the signal, which is in a spread modulated state, is demodulated. That is, the despreading unit 12 despreads the matched-filtered, received signal S1 by a PN code generated from a PN code generator 13.
The resultant despread signal is then sent to a correlator 14 which, in turn, accumulates the received signal for a certain period of time, Td. A threshold comparing unit 15 receives an output from the correlator 14. The threshold comparing unit 15 compares the output signal from the correlator 14 with a predetermined threshold value, thereby checking whether or not the PN code generated from the PN code generator 13 is synchronized with the spread PN code of the received signal S1. Based on the checked result, the threshold comparing unit 15 determines whether or not a desired PN code is acquired. That is, when it is determined that the output signal from the correlator 14 is higher than the threshold value, it is determined that acquisition of a desired PN code is achieved. When it is determined that the output signal from the correlator 14 is not higher than the threshold value, it is determined that acquisition of a desired PN code has failed. In the latter case, a PN code update signal is applied to a voltage control oscillator (VCO) 16 so as to hold (i.e., delay) the PN code sequence by one chip. Thus, PN code updating is conducted. Thereafter, the above procedure is repeated.
Meanwhile, if the period of the PN code is too long, the despread and correlation may be then conducted for the entire stage of the PN code sequence in the worst case. In such a case, the PN code acquisition time may greatly affect the system performance. Practically, in CDMA IS-95 systems, it is very important to achieve a fast PN code acquisition because they use a maximal linear PN code generator adapted to generate a PN code sequence with a period of 32768 (215) chips at most.
FIG. 2 is a schematic view illustrating the entire period of a PN code sequence in the form of a circle in order to facilitate the explanation of updating the PN code sequence. In FIG. 2, xe2x80x9cPN[i]xe2x80x9d represents a stage offset from an initial stage xe2x80x9cPN[0]xe2x80x9d by I chips. The term xe2x80x9cone chip holdxe2x80x9d means a procedure for offsetting a generated PN code sequence, based on an input signal, from a PN code generator by one chip when it is determined that PN code acquisition has failed at a stage xe2x80x9cPN[i]xe2x80x9d, thereby shifting the stage xe2x80x9cPN[i]xe2x80x9d to a stage xe2x80x9cPN[i+1]xe2x80x9d, and then again conducting a PN code acquisition procedure for the input signal at the stage xe2x80x9cPN[i+1]xe2x80x9d.
In the apparatus shown in FIG. 1, the VCO 16 is controlled based on the PN code update signal generated from the threshold comparing unit 15 in order to control, via hardware, the generation of input clocks, namely, the chip rate, such that the generation of input clocks is prevented for one chip. That is, the PN code generator 13 is controlled such that its output has an offset corresponding to one chip.
FIG. 3 is a schematic view illustrating a retard resulting from a PN code sequence update operation carried out in the conventional apparatus.
Problems occur when a PN code sequence update is carried out in the above mentioned manner. That is, the processing of input data is conducted for newly input data after the PN code generator shifts in stage from the current stage xe2x80x9cPN[i]xe2x80x9d to the next stage xe2x80x9cPN[i+1] while ignoring data received during the threshold comparison which is conducted after a correlation for N chips at the current stage xe2x80x9cPN[i]xe2x80x9d. However, this operation results in an unnecessary delay. Where a PN code sequence to be sought has an offset of N chips from the initial stage, a delay corresponding to N chips occurs. If xe2x80x9cNxe2x80x9d is large in this case, the PN code acquisition is then significantly delayed. This results in a great degradation in the entire system. Furthermore, in direct sequence spread spectrum systems, PN code acquisition is conducted, prior to a carrier recovery, in the demodulation operation. For this reason, the input signal is subjected to a square processing after the spreading and correlation thereof, thereby removing frequency phase components therefrom. That is, a non-coherent synchronization is conducted. Due to such additional operations, the time taken until the threshold comparison for one PN code sequence is lengthened. Moreover, the delay time is further lengthened where it is determined that acquisition of a desired PN code has failed. When the correlation, square processing, and threshold comparison are conducted using a digital signal processing unit, the delay time is also considerably lengthened.
Accordingly, an object of the present invention is to provide a PN code acquisition apparatus capable of minimizing the delay time involved in the updating of a PN code, thereby achieving a reduction in PN code acquisition time.
In accordance with the present invention, this object is accomplished by providing a pseudo noise code acquisition apparatus in a receiver adapted to receive a spread spectrum modulated signal comprising: a pseudo noise code generator for generating a pseudo noise code; a voltage control oscillator for adjusting a pseudo noise code sequence of said pseudo noise code generator; a despreading unit for despreading a spread spectrum modulated signal received in said apparatus in sync with said pseudo noise code; a correlator for accumulating said despread signal for a predetermined number of chips, thereby deriving a correlation value while sending, to said pseudo noise code generator, a control signal adapted to hold said pseudo noise code sequence by one chip every time a correlation for said predetermined number of chips is completed; and a threshold comparing unit for comparing said correlation value output from said correlator with a predetermined threshold value, thereby determining whether or not acquisition of a desired pseudo noise code is achieved, said threshold comparing unit generating a control signal adapted to advance said pseudo noise code sequence by one chip when it is determined that acquisition of a desired pseudo noise code is achieved.
In this apparatus, a PN code update is conducted prior to the threshold comparison after the spread spectrum signal is subjected to despreading and correlation for a predetermined number of chips. By this configuration, when it is determined, based on the result of the threshold comparison, that acquisition of a desired PN code has failed, it is possible to receive data associated with a new PN sequence without any retard. That is, the present invention eliminates a time delay required to remove data despread and correlated during the threshold comparison while receiving data associated with a newly updated PN code sequence.